Airbag devices for automobiles are installed in a large number of vehicles as a means for protecting the drivers and passengers from the impacts of collisions, and such airbag devices include airbag devices used for the front-passenger seats. Some airbag devices for the front passenger seats are each configured to be covered by a panel so-called “seamless instrument panel,” which is disposed in front of the front-passenger seat in a laterally long state, and which includes an airbag bulging door formed of a series of planar surfaces to prevent a to-be-torn line from being seen from the outside.
Among the front-seat passenger airbag devices disposed with the instrument panels of automobiles, there are known airbag devices each including a case member having a box shape and housing a folded airbag and an inflator configured to inject gas to inflate this airbag, and having an upper opening portion covered by a cover member integrally formed with the instrument panel, i.e., a seamlessly formed seamless design cover member. When the inflator blows an inflation gas into the airbag at the time of collision of an automobile, the airbag pushes up the instrument panel covering the bulging side of the airbag, and this pressure of the inflation tears the tear line provided to the instrument panel, thereby opening the door flap, i.e., the door portion.
Such a case member of an airbag device is formed by additionally attaching a door reinforcement portion to an inner portion including a square-tube shaped main body provided with a flange portion. In addition, after an outer portion to be integrated with an instrument panel is fixedly attached to this door reinforcement portion, the airbag and inflator held by a bottom plate member called a retainer, which is made of steel plate, for example, are inserted into the main body, and the case member is fixed to a fixing target portion such as a steering member using bolts or the like (e.g., see Patent Literature (hereinafter, referred to as “PTL”) 1 and 2).
In addition, in order to obtain the texture harmonized with a surrounding interior material made of a polypropylene resin, for example, such a seamless instrument panel typically includes a surface skin member made of the same kind of resin disposed on the top surface side of the panel. In addition, a soft-resin made reinforcement member is combined to the undersurface side of the panel so that the surface skin member can form a predetermined door shape with inflation and expansion of the airbag.
PTL 3 discloses an airbag device of this kind, for example. The reinforcement member integrally reinforces a to-be-door portion and a circumferential portion of the to-be-door portion along the surface skin member over a predetermined width. Adding a predetermined pressing force to the to-be-door portion placed over the reinforcement member with relative vibration generates heat due to the sliding friction between the to-be-door portion and the reinforcement member. This heat causes the resin of one or both of the to-be-door portion and the reinforcement portion to melt, thereby making it possible to bond the to-be-door portion and the reinforcement member together.
The welding using the sliding frictional heat, so called vibration welding requires that the welding targets be superimposed one on top of the other while a pressing force is added to the abutting surfaces of the welding targets. For this reason, the airbag on the reinforcement member side during assembly needs to be incorporated after the welding process. Therefore, the housing portion integrally provided to the reinforcement member has an opening at the bottom and is formed in a box shape without lid, and the bottom plate member to which the airbag has been attached is mounted to the seamless instrument panel to which the to-be-door portion and the reinforcement member have been surely welded.